kconfig: handle format string before calling conf_message_callback()

As you see in mconf.c and nconf.c, conf_message_callback() hooks are
likely to end up with the boilerplate of vsnprintf().  Process the
string format before calling conf_message_callback() so that it
receives a simple string.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Dirk Gouders <dirk@gouders.net>
diff --git a/scripts/kconfig/confdata.c b/scripts/kconfig/confdata.c
index d1216e4..629ad32 100644
--- a/scripts/kconfig/confdata.c
+++ b/scripts/kconfig/confdata.c
@@ -43,16 +43,16 @@ static void conf_warning(const char *fmt, ...)
 	conf_warnings++;
 }
 
-static void conf_default_message_callback(const char *fmt, va_list ap)
+static void conf_default_message_callback(const char *s)
 {
 	printf("#\n# ");
-	vprintf(fmt, ap);
+	printf("%s", s);
 	printf("\n#\n");
 }
 
-static void (*conf_message_callback) (const char *fmt, va_list ap) =
+static void (*conf_message_callback)(const char *s) =
 	conf_default_message_callback;
-void conf_set_message_callback(void (*fn) (const char *fmt, va_list ap))
+void conf_set_message_callback(void (*fn)(const char *s))
 {
 	conf_message_callback = fn;
 }
@@ -60,10 +60,15 @@ void conf_set_message_callback(void (*fn) (const char *fmt, va_list ap))
 static void conf_message(const char *fmt, ...)
 {
 	va_list ap;
+	char buf[4096];
+
+	if (!conf_message_callback)
+		return;
 
 	va_start(ap, fmt);
-	if (conf_message_callback)
-		conf_message_callback(fmt, ap);
+
+	vsnprintf(buf, sizeof(buf), fmt, ap);
+	conf_message_callback(buf);
 	va_end(ap);
 }